In cellular environments, such as LTE (Long Term Evolution) standardized in the 3GPP (3rd Generation Partnership Project), it is assumed that a plurality of base stations are disposed, where each base station communicates with terminals within its communication area. The communication area is generally referred to as a cell, which may be divided into a plurality of sub-regions by imparting directivity to an antenna of the base station. The sub-region will be referred to as a sector cell herein. In the following description, the term cell refers to a sector cell.
Generally, in LTE, the same wireless bandwidth is used among cells. Therefore, when performing transmission using the same wireless resource as that for an adjacent cell, a cell will experience strong interference from the adjacent cell (which will be referred to as inter-cell interference hereinbelow) regardless of whether the link is an uplink or downlink.
For example, in an uplink, a terminal located in the proximity of the base station having even a modest transmission power still has communication path quality that is not degraded much because of a large difference between a desired signal received by the base station and a level of interference by the adjacent cell. On the other hand, a terminal located in the proximity of the border of a cell poses a problem that it has significantly degraded communication path quality especially when a terminal in an adjacent cell located in the proximity of the border of the cell performs transmission using the same wireless resource at the same time, because of a smaller difference between a desired signal and a level of interference by the adjacent cell.
The same applies to a downlink, wherein when the transmission power is constant among cells, for example, a terminal located in the proximity of the border of a cell similarly poses a problem that communication path quality is significantly degraded because of a smaller difference between a desired signal to be received by the terminal and a level of interference by the adjacent cell.
Thus, the inter-cell interference issue is experienced regardless of whether the link is an uplink or a downlink. Moreover, since wireless communications also encounter shadowing, which is a variation of the intensity of radio waves, by blocking or reflection of radio waves by buildings, etc., cells that do not geographically lie next to a current cell may become an “adjacent” cell in terms of inter-cell interference.
FIG. 1 is an exemplary overview of a wireless communications system. A base station BS1 manages three cells C11, C12, C13, and base stations BS2-BS4 each likewise manage a plurality of cells. Communication is established between terminals located in the cells. In the illustrated example, a terminal UE1 is contained in a cell C11, and a terminal UE2 is contained in a cell C32. FIG. 2 shows the cell C11 in FIG. 1 and the actual shape of its adjacent cells C12, C13, C23, C32, C33, C42, taking account of the directivity of an antenna. Ovals indicate coverage of radio waves according to the directivity of antennas C12_1, C13_1, C23_1, C32_1, C33_1, C42_1. An overlapping area of ovals contains a cell border, and hatched hexagons represent actual cells.
A promising technique for solving the inter-cell interference issue is application of ICIC (Inter-cell Interference Coordination) in LTE (see NPL 1, for example). NPL 1 provides a statement that an object of ICIC is to control interference between adjacent cells, and information, such as resource usage or traffic load, from other cells should be taken into account.
Moreover, one method for implementing ICIC is an FFR (Frequency Fractional Reuse) technique (see NPL 2, for example). A fundamental operation of FFR will now be described.
First, a priority bandwidth is set in each cell so that it is different from those of adjacent cells. Next, a terminal reports its communication path quality information to a base station. The base station uses the communication path quality information to determine whether the terminal is less affected by inter-cell interference (which will be referred to as a center terminal) or more affected by inter-cell interference (which will be referred to as an edge terminal). In a case that the terminal is determined as an edge terminal, the base station limits an available bandwidth that can be allocated to the terminal to the priority bandwidth of the own cell. A bandwidth available to a center terminal is not limited. A scheduler allocates a wireless resource depending upon communication path quality from the bandwidth available to each terminal.
FIG. 18 shows an exemplary available bandwidth within each cell when such an FFR technique is applied to a case shown in FIG. 2. It is assumed that the bandwidth is managed by a PRB (Physical Resource Block) number, and a priority bandwidth for each cell is statically set beforehand. The available bandwidth is divided into three sub-groups, PRB1-3, PRB4-6, and PRB7-9, and in an area A0 for a center terminal, all bandwidths PRB1-9 are available. In other areas for edge terminals, any one of the three PRB sub-groups is set as the priority bandwidth.
For example, in an area All for an edge terminal in the cell C11 in FIG. 2, PRB1-3 are used as the priority bandwidth. By setting the priority bandwidth so as not to overlap between adjacent cells as in FIG. 18, interference between adjacent cells can be suppressed. Since communication path quality for an edge terminal using a priority bandwidth is improved, improvement of throughput for the edge terminal can be expected.
On the other hand, it is possible to dynamically set the priority bandwidth. In this case, it is necessary to notify the priority bandwidth among base stations. As a method of notifying the priority bandwidth among base stations, LOAD INFORMATION is specified (see NPL 3, for example). The uplink can be notified through HII (High Interference Indication) and the downlink can be notified through RNTP (Relative Narrowband Tx Power). The information notified through HII or RNTP can be created for each PRB number, the PRB being a smallest bandwidth unit for allocation to a user channel. For example, HII of a PRB to be defined as the priority bandwidth is set to one. NPL 3 provides a statement that “‘one’ indicates high interference sensitivity,” so that a PRB number for a terminal susceptible to inter-cell interference is thus notified. Likewise, RNTP of a PRB to be defined as the priority bandwidth is set to one. NPL 3 provides a statement that “‘one’ indicates no promise on the Tx power is given,” so that a PRB number is thus notified without a promise to allow for interference to an adjacent cell.